The present invention relates generally to Time-Reversal Acoustics (TRA) systems used to focus acoustic waves for various useful applications in the biomedical area. More particularly, the systems of the invention include an acoustic transmitter and an implanted or percutaneously inserted acoustic receiver. The receiver is configured to generate a useful electrical signal in response to receiving an acoustical signal from the transmitter. In addition, the receiver is configured to emit an electromagnetic wave (also referred to as radiofrequency or RF) signal to the transmitter. Such electromagnetic wave signal may be used as a feedback signal for tuning time-reversal acoustic system to focus acoustic waves at the location of such receiver as well as to transmit other pertinent information back to the transmitter. The system may be used for various useful purposes, such as cardiac pacing, neurostimulation or charging a battery of an implant system including the receiver. The device and method of the invention may be used advantageously as part of a medical instrument inside a patient's body as well as for other applications described below in more detail.
For the purposes of this description, the term “patient” is used to describe any person, animal, or other living being in which the medical instrument is inserted temporarily or implanted on a permanent basis. The term “medical instrument” or just “instrument” is used to describe various medical inserts and implants such as but not limited to needles, various scopes of flexible or rigid nature, implants, stents including drug-eluting stents, pacemakers and parts thereof, implantable electrical stimulators of all kinds including neurostimulators, neuromodulation devices, vagus nerve stimulators, hypoglossal nerve stimulators, thalamus stimulators, sacral nerve stimulators and spinal cord stimulators, implantable hearing aid devices including inner ear microtransmitters, cannulas, balloons, probes, guidewires, trocars, sensors, markers, infusion pumps, various implants functioning from an internal battery, and local medication delivery devices.
Electrical stimulation of nerves, nerve roots, and/or other nerve bundles for the purpose of treating patients has been known and actively practiced for many decades. Application of an electrical field between electrodes to stimulate nerve tissues is known to effectively modify signal pathways both with unidirectional and bidirectional stimulation along the nervous system to signal the brain or to signal organs to alleviate symptoms or control function. These applications are currently practiced with both externally applied devices and implanted devices. For example, applying specific electrical pulses to nerve tissue or to peripheral nerve fibers that corresponds to regions of the body afflicted with chronic pain can induce paresthesia, or a subjective sensation of numbness or tingling, or can in effect block pain transmission to the brain from the pain-afflicted regions. Many other examples include electrical stimulation of various branches of the vagus nerve bundle for control of heart rate, mediating hypertension, treating congestive heart failure, controlling movement disorders, tremors, treating obesity, treating migraine headache, and effecting the urinary, gastrointestinal, and/or other pelvic structure in order to treat urgency frequency, urinary incontinence, and/or fecal incontinence. Still other branches of the vagus nerve have been used to treat neuropsychiatric disorders. Additionally, applications are also known for electrical stimulation of nerves and nerve bundles in many other specific, selected nerve regions: for example, the pudendal or sacral nerves for controlling the lower urinary tract.
Neurostimulation may also be useful in treating a variety of other diseases including depression, paralysis, sleep apnea, angina, digestive tract disorders, Alzheimer's, obsessive-compulsive disorder, Parkinson's, epilepsy, accelerated healing of strains and tears, bone regrowth/repair in fractures, pain-pumps for intrathecal baclofen administration for spasticity, pain-pumps for intrathecal opioid administration for chronic neuropathic pain syndromes, spinal cord stimulators for failed back syndrome and cancer-related pain, neuropathic pain syndromes (e.g., herpetic neuralgia, phantom-limb pain—especially for blast/rocket victims), traumatic brain injury, and many others.
Depending on the individual patient, direct nerve stimulation can effectively modify signal pathways along the nerve, to and from the brain, and to and from organs in the body and thus provide relief of symptoms or control of bodily function. Treatment regimens and targeted nerve locations are known in related art through use of current, common stimulation devices and methods. Commonly implanted devices for nerve stimulation are made by such companies as Cyberonics, Medtronic, Advanced Bionics, and others.
Devices to provide such electrical stimulation may in some cases be applied externally, or in other cases it is more advantageous to implant or percutaneously insert all or part of the device. This invention pertains to devices and systems in which at least one portion providing direct electrical stimulation to the body tissue is either permanently or temporarily implanted or inserted. Such devices may include pacemakers, implantable defibrillators, neurostimulators and other devices for stimulating cardiac and other tissues.
Electrical energy sources connected to electrode/lead wire systems have typically been used to stimulate tissue within the body. The use of lead wires is associated with significant problems such as complications due to infection, lead failure, and electrode/lead dislodgement. The use of leads to accomplish tissue stimulation also limits the number of accessible locations in the body, as well as the ability to stimulate tissue at multiple sites (multisite stimulation). For instance, the treatment of epilepsy may require a minimum of perhaps 5 or 6 stimulation sites. Other diseases, such as Parkinson's disease, may benefit from more stimulation sites than the two utilized in current systems.
Beyond the problems of outright failure and placement difficulties, present day pacemaker leads inherently cause problems for pacemaker systems by acting as antennae, coupling electromagnetic interference (EMI) into the pacemaker electronics. Particularly problematic is interference with cardiac electrogram sensing and signal processing circuitry. With the exponential rise in the number of cellular telephones, wireless computer networks, and the like, pacemaker lead induced EMI will continue to spur increased complexity in the design of, and require significant testing of pacemaker devices.
Prior art describes various systems and methods for using acoustic energy to wirelessly energize an implanted component in order to generate a useful electrical signal inside the body of a patient. Examples of such systems may be found in the following US Patents and US Patent Applications, which are incorporated herein by reference in their entireties:
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Prior art devices typically include an acoustic transmitter and an acoustic receiver. The transmitter may be located inside or outside the body and the receiver is a small implantable or inserted component placed at or near the internal organ or tissue, which can benefit from direct electrical stimulation or another application of a useful electrical signal. The electrical signal is typically generated by the receiver using the acoustic energy received from the transmitter.
A key limitation of this arrangement is that the acoustic energy is unfocused and therefore is mostly dissipated in the surrounding tissues. Only a small portion of the acoustic energy is used for the purpose of generating a useful electrical signal. Because of that, the system has to be configured to rely only on small electrical energy available from the receiver or to transmit excessive acoustic energy which may jeopardize surrounding tissues.
Some systems of the prior art have suggested using phased array ultrasound transducers as part of an acoustic transmitter in order to focus ultrasound energy at the receiver location. This approach is of course better than any unfocused energy transmission but it too has a number of important limitations:                Location of the receiver has to be known in advance, which may not be easy to obtain;        Receiver has to remain in the same location, which is difficult to control due to breathing and other natural tissue movements;        A large number of individual transducers in the array is needed for effective focusing of ultrasound making the device complicated, large, and expensive;        Accurate predictive modeling of ultrasound waves passing through various types of soft tissues and bones is needed for the system to work effectively so that signals from all transducers converge on a single point where the receiver is located. Shifting tissues and inaccuracy of modeling make focusing less reliable.        
Focusing of ultrasonic waves using a concept of Time-Reversed Acoustics (TRA) provides an elegant possibility of both temporal and spatial concentrating of acoustic energy in highly inhomogeneous media. It was initially developed by M. Fink of the University of Paris. The TRA technique is based on the reciprocity of acoustic propagation, which implies that the time-reversed version of an incident pressure field naturally refocuses on its source. The general concept of TRA is described in a seminal article by Fink, entitled “Time-reversed acoustics,” Scientific American, November 1999, pp. 91-97, which is incorporated herein by reference. U.S. Pat. No. 5,092,336 to Fink, which is also incorporated herein by reference, describes a device for localization and focusing of acoustic waves in tissues.
An important issue in the TRA method of focusing acoustic energy is related to obtaining initial signal from the target area. It is necessary to have a beacon located at the desired tissue location to record and provide an initial signal from the focal region. In the TRA systems described in the prior art, most commonly used beacon is a hydrophone placed at the chosen target point. Other disclosed beacons may include highly reflective targets that provide an acoustical feedback signal for TRA focusing of acoustic beam. The need to have a beacon in the target region limits the applications of TRA focusing methods.
While scattering and numerous reflections from boundaries are known to greatly limit and even completely diminish conventional ultrasound focusing, in TRA they lead to the improvement of the focusing results. Fink et al. have demonstrated a remarkable robustness of TRA focusing: the more complex the medium, the sharper the focus.
The advantages of the TRA-based focusing systems over conventional ultrasound focusing are numerous:                TRA focusing approach is capable to precisely deliver ultrasound energy to the chosen region regardless of the heterogeneity of the propagation medium, for example behind the ribs or inside the skull. The ability to effectively localize ultrasound energy and avoid exposure of surrounding tissues to high levels of acoustic energy passing therethrough is important in many medical applications including ultrasound surgery and ultrasound-enhanced drug delivery;        TRA focusing systems may produce more effective spatial concentration of ultrasound energy than traditional systems; the focus volume can approach ultrasound diffraction limit, it can have a shape of a sphere rather than an elongated ellipsoid typically formed by most traditional focusing systems;        TRA focusing system may produce pulses with arbitrary waveforms in a wide frequency band. Ability to generate various waveforms is important in many applications, for example for optimizing the outcome of the ultrasound-enhanced drug delivery where the main mechanism of ultrasound action, sonoporation, is related to cavitation; the threshold of cavitation depends strongly on frequency and the form of the applied signal.        
Several examples of TRA focusing systems employing a passive ultrasound reflector or an active ultrasound emitter as a TRA receiver are described in the U.S. patent application Ser. No. 10/370,134 (US Patent Application Publication No. 2004/0162550) and U.S. patent application Ser. No. 10/370,381 (US Patent Application Publication No. 2004/0162507) to Govari et al. as well as a European Patent Application No. EP1449564, all of which are incorporated herein by reference. Described in these patent documents is a TRA-based high intensity ultrasound system designed for isolation of pulmonary veins. The receivers are implanted piesotransducers designed to reflect or emit ultrasound signal to be detected by an array of external transducers. In case of an active beacon, the electrical energy is typically delivered thereto via electrical leads from the control unit. The electrical energy is converted by the active beacon into the acoustic energy and transmitted to the outside of the body where it is picked up by outside sensors to determine the exact location of the receiver. In some cases, wireless circuitry and method of energy transmission is used to transmit the electrical energy to the active beacon, where it is then converted to the acoustic energy and emitted by the receiver. Alternatively, the receiver may comprise a passive ultrasound reflector, such as the one having certain geometry to produce a sharp and easily distinguishable ultrasound signature.
The need exists for an acoustically-powered system capable of delivering electrical energy to power an implantable electrical circuit. Such circuit may then be used as a leadless implantable tissue stimulation electrode, physiological sensor or a charger for an implantable battery.